Downtown Oakland sits on an unusual bit of geology — a large dune field mapped as the Merritt Sand. San Francisco is famous for its sand dunes, of course, and Points Reyes and Año Nuevo have some too, but the dune fields of Oakland and Alameda are the only ones within the bay. Here they are, labeled “Qds” on USGS Map OF-00-444, which shows the young (Quaternary) deposits of the Bay area.

They’re just like the dunes in San Francisco. They formed during the ice ages, when the shoreline was out near the Farallon Islands, the Bay was totally dry and the Sierra was full of permanent glaciers (on not quite the same schedule as the great continental ice caps). The rivers carried huge amounts of fresh-ground rock dust from the glaciers to the Bay and beyond, and the Pacific beaches of the time must have been formidable. Think of the brisk summer days we have when the sea wind is being sucked into the Central Valley, and now multiply that. Those winds blew all that sand here.

Brian Atwater of the U.S. Geological Survey mapped ice-age deposits in the Delta, and in 1982 he summarized the overall picture as “a stage on which three related and repetitious plays are presented simultaneously. In one play, wetlands and flood plains appear and expand as tidwater invades from the west, then become sites of erosion after the tidewater retreats. In another play, glacially eroded detritus from the Sierra Nevada builds alluvial fans and, reworked by wind, creates extensive sand dunes. In the third, little-understood play, streams draining the Sierra Nevada foothills and Coast Ranges episodically build alluvial fans. Spanish- and English-speaking persons enter during a major incursion of tidewater and find most of the stage covered with tules.”

Our dunes may sit higher than the buried dunes of the Delta because the conditions that built them were more stable here. There was always a wind gap at the Golden Gate and always lots of sand available on the other side.

In detail, the Merritt Sand (Qms) reaches the edge of Lake Merritt.

It differs from the marine terrace deposits (Qmt) that I’ve described before. It consists of very fine sand, with no clay. It’s also higher and less flat. Apparently the original, undisturbed surface featured yardangs — elongated ridges of sand running in the direction of the wind — whereas the dunes of Alameda were the more typical arc shapes known as barchans. All of that is obliterated today.

You can see the edge of the Merritt Sand platform from across the lake where the streets rise abruptly away from the shore. Snow Park is probably the least disturbed exposure.

Another exposure stands out between Jackson and Madison streets, although it probably has also been excavated.

It’s the back yard of an apartment building at 160 17th Street. The view is nice from there.

That same agreeable elevation attracted Oakland’s early elite, who put up a row of mansions overlooking the lake. Of those great homes, only the Camron-Stanford House survives.

The second geology ramble — my name for a long walk that starts in one place and ends in another — is a long and rugged one, just to show you I’m not kidding about these. From the Rockridge BART station to the Orinda BART station is a walk of more than 9 miles with a thousand-foot climb in the middle.

There are several ways to do this. This summer I’ve pioneered what I’ll call the middle route on two separate outings. Here they are, first on Google Maps and then on the geologic map (both images are 1000 pixels). The photos are a mixture from both traverses.

From the Rockridge station, the route along the south side of Route 24 is more direct while the alternative, up Chabot Road to Roanoke Road to The Uplands to Tunnel Road (dashing across Tunnel to the uphill side), takes you through more shade and past more rocks, starting on Roanoke.

The mixed lithologies of Franciscan melange (KJfm) give way to rugged outcrops of the Leona rhyolite (pink color) as you cross Vicente Creek on Tunnel Road. Admire them at the century-old estate called The Rocks. Beyond the Fire Garden is a short stretch without sidewalks that passes a long, excellent exposure of Leona Rhyolite. This was quarried in the 1930s and again in the 1950s during construction of upper Broadway, the Caldecott Tunnel and Route 24.

The rocks change to mudstone of the Great Valley Sequence (Ku), then the much younger Sobrante Formation (tan color), as you ascend Tunnel Road’s steady, gentle grade.

The change to the Claremont chert is dramatic as you near the ridgetop and enter Sibley Preserve.

Cross the park on the Round Top Loop trail, which goes through the coarse-grained sedimentary rocks of the Orinda Formation (Tor), although you won’t see much of them. Take the Volcanic Trail left, which leads into the structurally overlying basalt of the Moraga Formation (Tmb). The quarry that carved up these golden hilltops extracted that basalt. In 0.2 miles, at the right edge of this photo, is a road with a cattle gate that exits the park.

The views change dramatically on the east side of the hills, whether you’re looking to the left up Siesta Valley . . .

. . . or to the right toward Mount Diablo.

Straight ahead lies the unbuilt Wilder Ranch subdivision of Orinda. The valley it sits in is the continuation of Siesta Valley, and it’s underlain by nonmarine sedimentary rocks of the Siesta Formation.

Both valleys owe their shape to the large fold, or syncline (“sloping together”), in the Siesta Formation that’s noted on the geologic map. The Moraga Formation basalt is also downfolded by this syncline, and it crops out again in the hill with the quarry scar.

This subdivision looks bleak, but the developers are doing the job right. The lots are gray because they’re sealed with some tough, pliant substance that prevents all dust and weeds. And as you cross, the route takes the dirt road running from the intersection of Wilder and Bigleaf Roads to the big bend in Rabble Road. You’ll pass several vegetated catch basins designed to hold the extra runoff from the new properties.

The route goes from Rabble Road to Boeger Ranch Road, but take the straight spur between them and follow it to the end, where a footpath connects with the end of Oak Road. All of this area is mapped as mudstone of the Mulholland Formation, of which I know nothing beyond its (young) age, Miocene and Pliocene. From Oak, take Stein Way down to busy-busy Moraga Way and from there head to the BART station. Sidestep as much of Moraga Way as possible by taking Camino Encinas.

If time permits, stop for a beer at The Fourth Bore in Theatre Square. If you take this ramble the other way, stop for a beer at Ben & Nick’s. Either way, you’ve earned it.

The first time I made this trek, many years ago, I took the northern route: up Claremont Canyon, north on the Skyline Trail, then down through the Lomas Cantadas maze to Camino Pablo. That was work. I’ve hiked up the canyon on Claremont Avenue several times, but the traffic is nerve-racking. The alternatives, through the Hiller Highlands or Grandview neighborhoods, are steep, sunny trudges. On the Orinda side it would be more fun to descend through the East Bay MUD land from the Skyline Trail (for which you need a hiking permit). I plan to attempt the northern route again when the weather cools. I have a vague scheme for a southern route, too.

More than once on this blog, including last week, I’ve described streams as sleeping creatures that wake up in floods. Kittens that turn into tigers, nebbishes that become the Incredible Hulk, pick your own metaphor — streams do most of their geological work in spasms. The downpour that happens once in a century, filling a creek and scouring its streambed a bit deeper and wider, is one hour in a million hours.

Oakland’s Lake Merritt is prone to floods as well as high tides. The tides are slowly growing as sea level rises, of course, but the greater threat of flood comes from the land side. Oakland’s soil absorbs much less rainwater than it used to. As Oakland grew, its dusty streets were sealed under asphalt, its grassy lots occupied by homes with guttered roofs, its footpaths paved over in concrete. Today these impervious surfaces shed the rain, and the runoff drains swiftly away to the nearest body of water.

In October 1962 the Columbus Day Storm dumped over 4 inches of rain in a single day. Lake Merritt rose more than 7 feet and left the surrounding roads waist-deep in stormwater. A few years later a large flood-control station was installed at 7th Street that regulates the tidal lake.

We can do better on the land side. Greener building practices, sustained from now on, will gradually offset our disruption of the local hydrology. This is one of those — panels of pervious pavement flanking the street trees near the lake on East 18th Street. They’re made of a porous concrete that lets water through, like a super mulch. They let the tree roots breathe, too.

Another green practice you’ll see near the lake and elsewhere around town is rain gardens, shallow basins filled with vegetation that catch and absorb rain runoff before it can reach the lake.